The present invention relates to the art of protection circuitry for a power amplifier and, more particularly, for RF power amplifiers of the type employed in AM radio broadcasting.
It is known in the art to provide VSWR protection circuitry for use in protecting an RF power amplifier to insure that the power amplifier is not damaged at high levels of VSWR. Such protection circuits, for example, will sense the forward power level and the reflected power level and control the transmitted output power when the ratio of the reflected power to the forward power becomes too great. Such a circuit is disclosed in U.S. Patent to G. D. Miller, U.S. Pat. No. 4,353,037.
It is also known in the prior art to protect an RF power amplifier by turning the amplifier off for a short period in the presence of detected high VSWR conditions which may be caused by lightning. However, it has been determined that even after the transmitter has been turned off, energy stored in the output network of such an RF transmitter may cause RF current to flow for a short period. The frequency of this stored energy will not be exactly the same as the RF signal obtained from the RF oscillator which drives the power amplifier. Consequently, the current in the output network of the power amplifier will not be in phase with the drive applied to various transistors employed in the power amplifier. These transistors may take the form of MOSFET transistors connected together in a bridge circuit, such as that illustrated in my U.S. Pat. No. 4,580,111 entitled "Amplitude Modulation Using Digitally Selected Carrier Amplifiers", and which patent issued on Apr. 1, 1986. As will be discussed in greater detail hereinafter, the transistors in such amplifiers may fail during this operation.
It has been determined that failure of these transistors may be prevented by discontinuing the supply of the RF signal to the power amplifier when the amplifier is turned off and instead supplying to the RF amplifier a frequency signal having a frequency corresponding with that of any signal flowing in the output network.